INTERPRETING VARIATION IN WEATHERING COLOR IN ARCHAEAN CARBONATE ROCKS USING PETROGRAPHY, CATHODOLUMINESCENCE, AND GEOCHEMISTRY: AN INTEGRATED APPROACH

Carbonate strata of the 2.52 Ga Nauga Formation, Transvaal Supergroup, South Africa show repetitive cm-scale bedding. These cm-scale beds consist of microbially laminated dolomite capped by a thin interval of mechanically bedded carbonate and siliciclastic material. Despite similarities in lithology, this repetitive bedding often shows cm-scale alternation between grey- and brown-colored weathering, suggesting a potential for geochemical differences. Two 20 cm-thick samples, located at the interface between grey- and brown-weathering intervals, were collected near Prieska, South Africa. The paleoenvironmental settings should be remarkably similar in these stratigraphically adjacent samples, yet the differences in weathering suggest fundamental differences that might be reflected in the chemistry and diagenesis of the samples.

Here we present the results of an integrated petrographic, cathodoluminescence, and geochemical analysis of these samples. Photographic images of in situ differences in weathering profiles were used to construct stratigraphic columns and aid in more fine-scale investigation. Thick and thin sections have been compared for petrography and cathodoluminescence with the goal of identifying potential differences in microfabrics and texture of depositional phases. This integrated optical analysis allowed targeting of individual phases for geochemical analysis. Geochemical analyses were prepared from drilled and powdered rock which were analyzed using Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES) for major and minor elements and Isotope-Ratio Mass Spectrometry (IRMS) for oxygen isotopes to constrain the possible origin of distinct weathering color and to better understand this Archaean depositional environment.